An ethernet adapter is a service Resource. It's possible to uninventory services by going to the Browse Resources > Services page and adding &debug=true to the URL and loading that new URL (this causes an Uninventory button to be displayed). However, if the underlying managed resource (e.g. the ethernet adapter) still exists, it will get rediscovered and auto-reimported into inventory, so the univentory will have been for nothing. What is missing is a way to "ignore" discovered services. This is a known issue. For more details, see:

We don't have an ignore function - that's the enhancement request in the bugzilla issue I posted. We have an undocumented way to uninventory services, because it would actually be useful in the case where the underlying managed resource no longer exists (i.e. because the service will not get rediscovered).